OA from Kaplan -
Here, "swim to migrate" may sound awkward. This phrasing implies that the seals don't normally swim but only swim twice a year on their migration. A more logical phrasing would be that the "seals migrate by swimming." Then a grammar rule is violated with the pronoun "which" at the end of the underlined portion. A pronoun must unambiguously refer to one particular noun. Here, "some of which" could be referring to some of the elephant seals that regularly dive to great depths or to the North Pacific. The correct answer will fix the pronoun error and express the intended idea clearly.
Scan and Group the Answer Choices:
A vertical scan of the first words in the choices reveals a 3-2 split between "migrate" in (B), (C), and (E) and "swim to migrate" in (A) and (D). There is also a 2-2-1 split in the final words of the choices, between "some of whom" in (B) and (C), "some of them" in (D) and (E), and "some of which" in (A).
Eliminate Wrong Answer Choices:
"Swim to migrate" is the awkward construction noticed in the initial reading of the sentence, so eliminate (A) and (D). Alternatively, you might start by eliminating (B) and (C), which use "whom" to refer to elephant seals. On the GMAT, "whom" may not refer to animals, only people.
That leaves (E), which is correct because it subordinates the swimming to the main action of migrating and replaces "some of which" with the clearer "and some of them." The sentence now starts a new clause in which the plural "them" clearly refers to the subject of the first clause, the seals.
TAKEAWAY: It can be dangerous to pick an answer because it "sounds good," because the GMAT knows the kinds of wordings that sound good in real life but are grammatically incorrect. However, it is less dangerous to eliminate choices because they are awkward or place the sentence's emphasis on the wrong thing.